1. Field of the Invention
The invention relates generally to the field of instrumentation used in wellbores drilled through Earth formations. More specifically, the invention relates to methods and apparatus for communicating signals to an instrument in a wellbore from the Earth's surface.
2. Background Art
Instruments used in wellbores drilled into the Earth's subsurface include a wide variety of sensing devices and mechanical operating devices. Examples of the former include pressure and temperature sensors, inclinometers and directional sensors, capacitance sensors, fluid density sensors, among others. In using such instruments, it is often necessary to send signals from the Earth's surface to the instrument to affect instrument operation or to provide information that may be used in the instrument.
For instruments deployed in a wellbore using armored electrical cable (“wireline” deployment) signals are transmitted along the cable to the instrument from the surface, typically from a surface recording system. For instruments deployed using a drilling rig or similar apparatus, where the instrument may be conveyed at the end of a drill pipe or tubing string, it is known in the art to send signals to the instrument by modulating the flow of drilling fluid through the drill pipe. Such modulation may be detected and decoded at the instrument by a flow sensor or a pressure sensor. It is also known in the art to send signals to the instrument by modulating the rate of rotation of the drill pipe. See, for example, U.S. Pat. No. 6,847,304 issued to McLoughlin and U.S. Pat. No. 5,113,379 issued to Scherbatskoy. It is also known in the art to communicated signals to an instrument in a wellbore by modulating fluid pressure from the Earth's surface. See, for example U.S. Pat. No. 4,856,595 issued to Upchurch and assigned to the assignee of the present invention.
In some cases, it is impractical to use any of the foregoing techniques for communicating signals to an instrument in a wellbore. For example, using “slickline” (a solid wire or wire rope conveyance having no insulated electrical conductors) there is no practical way to send electrical signals to the instrument from the Earth's surface. Further, it is not possible to rotate an instrument from the surface when conveyed by slickline or by coiled tubing. Finally, some wellbore instruments are materially complicated as to design by including a pressure or flow sensor.